The history of pathogens can be construed as examining the known information regarding the occurrence of pathogens in Geologic time, and also the period of mankind's scientific discovery of these disease agents. Many pathogenic organisms show ancestral traces in the fossil record, as well as in carbonized insects (insect compressions formed in ancient sediments); however, detailed evidence of disease relationships are difficult to ascertain from such data. In any case, the progression of pathogens from non-hominid fauna to interactions with hominids can be traced in a general way through a variety of archaeological, historical ecology and other reconstructive techniques.
The co-evolution of pathogens with humans is a complex and important subject, especially since man is the only animal who has been capable of changing host conditions in a rapid manner by evolutionary standards (e.g. by widespread use of herbicides). The Red Queen Hypothesis has been advanced to explain the evolutionary arms race that is ostensibly present between all predators and their prey, as well as between pathogens and their hosts. The Red Queen Hypothesis applied to pathogens asserts that hosts must advance their species fitness to defend against a given pathogen, but that also the pathogen must evolve more clever functionality to successfully utilize that host species. Given the short life cycles of pathogens, that the long run macroevolutionary advantage may lie with the pathogen, since they have the ability to more rapidly transform their genes and hence efficiency, leaving humans as well as other mammals as well as plants at a disadvantage.
The fossil record has some evidence of the occurrence of pathogenic agents from hundreds of millions of years ago. Besides fossil evidence, it is logical that these lifeforms have been present on Earth for considerable time. Prokaryote ancestors of present day bacteria have been found in Gunflint Chert of northern Minnesota dating to approximately 1.9 billion years before present. These early organisms have a filamentary structure quite similar to many modern day bacterial species; however, there is no present method of knowing which, if any, of these early bacteria were actually pathogenic. As another example, flatworms, having no hard structures, are not found in the fossil record, but they are found as parasites in carbonized insects in the Pennsylvanian and Tertiary Periods. Since approximately 250 million years before present, these flatworms gave up any free-living existence and began to propagate as the present day flukes and tapeworms. Parasitic roundworms may be the most numerous of any faunal form of the present day, and these nematodes have also been found in carbonized insects as well as Quaternary mammals; free living organisms of this type have been found in Burgess Shale dating to the Cambrian.
This period is significant in that it represents the emergence of hominids, and thus their initial interactions with zoonotic pathogen reservoirs residing in other primates, ungulates, arthropods and other species. Certain pathogens such as the trypomastigote stage of trypanosome parasites carried by tsetse flies became established in Pleistocene ungulates, and this long term residence in hooved animals explains that groups relative resilience with respect to this disease. The presence of ungulates carrying tsetse (Glossina) associated diseases at Olduvai Gorge suggests the likelihood that these ungulates were the likely reservoir for tsetse to infect hominids in the mid to late Pleistocene.
Electron micrograph showing a Plasmodium sporozoite migrating
through cytoplasm. Source: Ute Frevert
A major element of pathogen evolution appears to have arisen during the mid to late Holocene, as urbanization began. One theory is that for certain zoonotic diseases, faunal reservoirs kept human affecting pathogens alive; a competing, but not mutually exclusive theory is that mutations of human pathogens arose, and were maintained as opportunities for close contact increased in urban settings; before this area of large, dense human population, certain pathogens may not have had an opportunity to thrive. This urbanization hypothesis helps in explaining why disease transmission was chiefly in the direction of Europe to the Americas, where more urban societies were settling into less densely populated cultural areas.
Livestock cultivation also accounts for certain co-evolution and increased incidence of pathogenic diseases in humans. An example of this phenomenon is the intensive use of the water buffalo in the Indus Valley and its introduction to new regions in Mesopotamia and the Nile Valley, leading to outbreak (and subsequent geographic spreading) of a skin leprosy disease in humans. More intense aggregations of beef cattle and their intimate contact with humans in mid-Holocene agricultural changes led to widespread occurrence of the beef tapeworm in the human population.
Malaria is thought to have originated in Africa about 10,000 years ago. By the 19th century, malaria had spread globally. Over one-half of the human population was at significant risk, with one in ten people affected and expected to die from it. Today, 350-500 million cases of malaria occur worldwide, claiming annual deaths in excess of one million.
Malaria is a mosquito-borne disease that can use non-human primates as biological reservoirs. It is caused by the parasite genus Plasmodium. Some of the different malaria parasite species include Plasmodium vivax, P. malariae, P. falciparum and P. ovale. Infected people often experience fever, chills and flu-like illness. If left untreated, severe complications may ensue,with death a likely outcome. Plasmodium unfortunately provides researchers with the opportunity to observe ongoing evolution of a human parasite in response to antimicrobial drugs.
It has been hypothesized that a number of modern day diseases may have been present in the early to mid Holocene, but were unable to spread in humans due to tribal societies living in small bands. It is possible that some of the most virulent of modern diseases, such as Ebola, simply killed off entire bands of early humans, but did not propagate, given the relative isolation of tribes; after extinguishing an entire localized band of humans, such a disease may have resided in other faunal reservoirs for millennia, until densities of humans were such as to render the disease more spreadable, and thus more noticeable.
Pathogenic disease has been deduced to have occurred in vegetation as early as the mid-Holocene, with pollen data from eastern North America illustrating a dramatic decline in Hemlock trees about 4800 years ago.
Some of the earliest significant Middle Ages history of pathogens occurred in Africa in the 8th century AD with advent of Arab colonization. The widely popularized subject of Bubonic Plague is more closely associated with Europe and Measles is most often discussed as a European and Americas issue.
The bubonic plague, or Black Death, epidemic appears to have begun in China in the early 1330s AD. An Italian merchant ship that had traded with Chinese merchants carried infected rats and crew-members dying of the plague to Sicily. From this inception, the disease spread rapidly throughout Europe. It is believed the plague killed 137 million people in a 400 year period. Presently, the World Health Organization reports that 1000 to 3000 cases of plague are reported each year.
Bubonic plague is caused by the bacterium Yersinia pestis. The cycle through which the plague is perpetuated begins when a flea bites an infected rat. The bacteria multiply within the flea’s gut, and the flea then transmits the disease to a human by biting. The most common form of infection in a human is a swollen and painful lymph gland that forms buboes, from which the disease is named.
References to measles can be found dating back to the 7th century AD. Some described the disease to be more dreaded than smallpox. By the 16th century, most people in Europe, Asia and North America had already been exposed to measles and thus were immune. As such, it chiefly became a childhood disease. The worst measles plague was when European traders and explorers transmitted measles, as well as smallpox, to the indigenous people of North and South America in the 1500s. These Early Americans had no resistance to the disease. About nine out of ten people in North and South America succumbed to measles and smallpox.
Measles is a highly contagious viral disease in the family Paramyxoviridae and the genus Morbillivirus. This disease can spread from monkey to man, man to man, man to monkey and monkey to monkey. Indications of measles include rash in the mouth and on the cheeks, neck, chest and body. Other complications may occur such as middle ear infection, bronchopneumonia and encephalitis.
The first recorded instance of this disease, attributed to bacteria of the genus Rickettsia, was documented in 1489 AD at the siege of Granada. This disease evolved or mutated from a less fatal form, apparently exacerbated by conditions of siege in crowded, dirty and rat infested conditions of the Muslim army at Granada. While the invading Arabs are attributed to have brought the disease into Spain, great casualites were inflicted on the Spanish army from this disease whose vectors include both lice and rats.
Cotton rat (Sigmodon hispidus), carrier of the hantavirus,
a pathogen only discovered in 1993. Source: CDC
Hippocrates seems to have been silent on the issue of microscopic disease elements, not surprisingly, since the world of microbes was not really even speculated in these early times.The first scientific exposition of pathogens is generally regarded that of Girolami Fracastoro. The first true convincing depiction of a minute pathogenic phenomenon was given later in the 17th century by the Italian Giovanni Cosimo Bonomo, who described the steps in pathogenic infection by a tortoise shaped mite; nevertheless, decades transpired after the late 16th century invention of the microscope and its 1660s perfection, before meaningful scientific understanding of these microscopic disease agents. Antoni van Leeuwenhoek (1623-1732) is credited as the first microscopist to catalog an vast array of microscopic organisms (e.g. protozoa, molds, bacteria) and other cells not visible to the naked eye. Agostino Bassi (1773-1857) conducted a landmark infection experiment by inoculating healthy silkworms with tissue from dead silkworms who succumbed to a disease known as muscardine; Bassi ascribed this disease transmission to a minute parasitic fungus.Influenced by Bassi, Johann Lucas Schoenlein (1793-1864) discovered another parasitic fungus responsible for ringworm.
The 19th century icon, Louis Pasteur, is best known for demonstrating that fermentation is the result of the growth of microorganisms and for perfecting microbiological techniques that are the precursors to modern surgical sterilization methods; however, his crowing achievement in pathogen science was the development of a vaccine for the virally transmitted disease of rabies. His contemporary Robert Koch, also contributed greatly to linking microbial infection with specific diseases, by introducing what are now referred to as "Kochs Postulates" -- four requirements for establishing cause and effect. Probably the most notable 20th century advance was Jonas Salk's 1955 discovery of a vaccine for the virally transmitted polio disease.
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- ^ T.D.Allison, R.E. Moeller and M.B.Davis. 1986. Pollen in laminated sediments provides evidence for mid-Holocene pathogen outbreak. Ecology, 67, 1101-5
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- ^ Arno Karlen. Modern history in plagues, disease, microbes and man
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- ^ Girolami Fracastoro. 1646. On Contagion
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